The conversion of plasminogen, an inactive serum zymogen, into a proteolytically active molecule, plasmin, has been shown to occur in cell cultures transformed by both RNA and DNA tumor viruses; chemically induced tumor cells; and human tumor cell lines. Normal, control cells grown under identical conditions apparently lack the requisite plasminogen activator. The objective of this proposal is to examine the surface membranes of transformed cells and determine how they are structurally and functionally modified as a consequence of plasmin formation. The proposed investigation will be facilitated by the ability to specifically and quantitatively remove plasminogen from serum by affinity chromatography. The resulting serum, devoid of plasminogen, provides a unique tool to directly probe the effects of a specific proteolytic activity on cells in culture. Polyacrylamide gel electrophoresis will be used to analyze the protein and glycoprotein composition of the surface membrane of transformed and normal cells grown either in the presence or absence of plasminogen. Surface alterations which occur as a direct result of plasmin formation will be correlated with the known properties of transformed cells in culture. The control and biochemical specificity of plasminogen activation by transformed cells will also be examined in detail. In addition, the proteolytic activity of the plasminogen activator itself, independent of zymogen activation, will be examined as a possible endogenous mediator of alteration on the cell surface.